Wireless spectrum is a limited resource. As WiFi™ wireless local area networks become more prevalent, there will be higher density deployments that suffer from more co-channel interference, and the need to improve bandwidth per cell.
Multi-User Multi-Input Multiple-Output (MU-MIMO) techniques of IEEE 802.11ac can improve downstream throughput by transmitting to multiple clients at the same time. However, this assumes that there is traffic for multiple clients on the same access point (AP) and there is sufficient separation to adequately null between the two or more clients. In most cases, the data rates used for MU-MIMO will be lower than those that can be used for single user MIMO because nulling is not perfect, degrees of freedom are used for nulling versus signal-to-noise ratio improvement, and the combined channels will change more frequently.
Distributed-Input Distributed-Output (DIDO) techniques have even more potential value, but conventional DIDO techniques involve the use of a data center for centralized signal processing that has its own set of challenges. Network bandwidth requirements to communicate the raw modulated signals to all of the APs involved in a DIDO transmission are substantial, and this is exacerbated as more transmit antenna paths are added at each AP to improve the receive signal-to-noise ratio at clients. Consequently, current DIDO techniques are not practical as wireless data rates increase, e.g., as with IEEE 802.11ac. In addition, latency in responding to changing channel conditions is also an issue that is made worse by a centralized processing DIDO solution.